The invention relates generally to in-situ inspection of coated components and, more particularly, to in-situ eddy current inspection of coated components in turbine engines, for example airfoils in gas turbine engines and coated turbo components in locomotive diesel engines.
A gas turbine engine typically includes a compressor that supplies pressurized air to a combustor. The air is mixed with fuel in the combustor and ignited to generate hot combustion gases. The hot gases flow downstream to one or more turbines that extract energy from the hot gases to power the compressor and provide useful work, such as generating power at a power plant or powering an aircraft in flight.
Each turbine stage typically includes a turbine rotor assembly and a stationary turbine nozzle assembly for channeling combustion gases into the turbine rotor assembly disposed downstream therefrom. The turbine rotor assembly 50 commonly includes a number of circumferentially spaced apart rotor blades 20 extending radially outwardly from a rotor disk 30 that rotates about the rotor 10, as illustrated schematically in FIG. 1. Rotor blades generally include airfoils (also indicated by reference numeral 20) and are commonly called xe2x80x9cbucketsxe2x80x9d for land based turbine engines. As used here, the term xe2x80x9cbladexe2x80x9d encompasses xe2x80x9cbucketsxe2x80x9d as well as blades. The rotor assembly is housed within a case 40.
The stationary turbine nozzle assembly 60 includes a number of circumferentially spaced apart stationary vanes 21 radially aligned with the rotor blades 20, as schematically illustrated in FIG. 2. The stationary vanes are disposed between inner and outer bands 42, 41. The stationary vanes include airfoils (indicated by reference numeral 21 in FIG. 2) and are configured to direct the hot combustion gases to the downstream turbine rotor assembly, and, more particularly, toward the rotor blades 20. Vanes are commonly called xe2x80x9cnozzlesxe2x80x9d for land based turbine engines, and as used here the term xe2x80x9cvanexe2x80x9d encompasses both vanes and nozzles.
An exemplary airfoil 20 is illustrated in FIG. 3 in cross-sectional view and includes a base metal 24, for example formed of nickel superalloys, such as GTD111 or IN738. The core can be hollow or solid. The core is coated for protection against erosion and to render the airfoil suitable for use in high temperatures, with exemplary protective coatings 22 being NiCoCrAlY or MCoCrAlY. In addition, the airfoil may also include an outer ceramic coating 23, to act as a thermal barrier (hereinafter a xe2x80x9cthermal barrier coatingxe2x80x9d).
In response to the stress induced by thermal gradients in the airfoils and other operating conditions in gas turbine engines, cracks can develop in the airfoil coatings. An exemplary crack 52 is indicated in FIG. 3. Although cracks generally terminate at the diffusion zone between the protective coating and the base metal, cracks do occasionally penetrate into the base metal.
In order to inspect airfoils for cracks, presently airfoils are removed from the rotor assembly and from the nozzle assembly during outage cycles for inspection, refurbishment, and determination of the remaining lives of the airfoils. The outage cycles occur about every 24,000 to 30,000 operational hours. In the current inspection process, the airfoils are first inspected by fluorescent penetrant inspection, to detect cracks in the coatings. When cracks are detected, the cracked airfoil is hand blended using a hand-held grinder to remove the cracks. A final fluorescent penetrant inspection is conducted to confirm that the cracks have been removed.
One drawback to the present airfoil inspection method is that removal of the airfoils from the rotor assembly and from the nozzle assembly and the subsequent fluorescent penetrant inspection of the airfoils are time and labor intensive, contributing to long and expensive gas turbine outages. In addition, the fluorescent penetrant inspection method detects only the presence of a crack and does not determine whether the crack is localized within the coatings 22, 23 or has penetrated the base metal 24, nor does the existing inspection method determine the depth of the crack. Moreover, the grinding performed while chasing the cracks progresses to the base metal in many instances, undesirably reducing the wall thickness of the airfoil.
Accordingly, it would be desirable to develop a method for in-situ inspection of gas turbine airfoils to determine the presence of cracks in the airfoils. It would further be desirable for the method to determine the crack depth and whether the crack has penetrated the base metal of the airfoil. In addition, it would be desirable for the method to employ nondestructive inspection techniques.
Briefly, in accordance with one embodiment of the present invention, a method for in-situ eddy current inspection of at least one coated component is disclosed. The coated component includes a base metal and a coating disposed on the base metal. The method includes applying a drive pulse at a measurement position on an outer surface of the coated component, while the coated component is installed in an operational environment of the coated component. The method further includes receiving a response signal from the coated component, comparing the response signal with a reference signal to obtain a compared signal, analyzing the compared signal for crack detection, and determining whether a crack near the measurement position has penetrated the base metal, if the presence of the crack in the coating is indicated.